Holography is a technology for recording and reproducing a three-dimensional shape of an object and has a step of recording the object shape and a step of reproducing the recorded object shape. A medium recording a three-dimensional shape of an object as interference fringes or a reproduced image is referred to as a hologram. A laser beam with strong coherence is used for recording and reproducing the hologram.
In recent years, a technology of digital holography for acquiring a hologram as a digital image has developed with development of an imaging device. In the digital holography, an imaging device is used instead of a recording medium, but a basic principle is the same as that of the prior-art holography. The digital holography has more merits than the prior-art holography. For example, development processing is not needed, and various types of image processing can be applied to an acquired hologram image. Phase information of object light can be calculated from the hologram image by using light-wave back propagation simulation by a computer or the like, and a three-dimensional shape of an object can be also reproduced with high precision.
However, the resolution (pixel pitch) and the number of pixels of an imaging device are physically limited and are much poorer than those of a recording medium in usual, and thus, depending on the size of a measurement target, a wavelength of the laser beam, and an optical-path length of an entire optical system, zero-order diffracted light and a virtual image might be overlapped with a real image, which extremely deteriorates a reproduced image. In order to solve this problem, a technology of phase shift digital holography for imaging a plurality of hologram images by changing a phase of reference light is proposed (see Patent Literature 1, for example).
Patent Literature 1 describes a device comprising: phase shift means (piezo element) for shifting a phase of reference wave; hologram imaging means for generating a hologram image by irradiation of object wave and the reference wave to an imaging area; a calculating means for executing various processing to the plurality of hologram images generated by the reference wave and the object wave obtained from each of a plurality of phase-shifted reference waves; and the like. According to the description in Patent Literature 1, a device of a phase shift digital holography capable of preventing the reproduced image from accompanying a ghost image without requiring off-axis (crossing angle of reference light and object light) can be provided.